Many industrial process residues contain vanadium in association with other inorganic constituents such as carbon and sulphur and the oxides of aluminum, silicon, iron, magnesium, calcium, sodium, potassium, nickel, cobalt and molybdenum. Such residues are often processed for recovery of the more valuable constituents, especially vanadium. Examples of residues of this type are spent hydrodesulfurization catalysts and fly ash and furnace bottom ash arising from the combustion of crude petroleum oil.
Vanadium is usually recovered by treating the residues with sulfuric acid which dissolves vanadium along with iron and other metals, separating undissolved material from the solution and then precipitating vanadium from the solution by oxidation followed by partial neutralization with ammonia and boiling, whereby there is precipitated a mixture of hydrous oxides of vanadium and iron known as "red cake." Such a process is described in a publication of William Whigham "NEW IN EXTRACTION: VANADIUM FROM PETROLEUM", CHEMICAL ENGINEERING, Mar. 1, 1965, pages 64-66. In order to obtain pure vanadium pentoxide it is necessary to effect further processing. For example, the red cake may by redissolved in sulfuric acid and the vanadium recovered by solvent extraction or liquid ion exchange. Such extraction processes are well known in the art; for example, one such process is described in "COMMERCIAL RECOVERY OF VANADIUM BY THE LIQUID ION EXCHANGE PROCESS" by R. R. Swanson, H. N. Dunning and J. E. House, ENGINEERING & MINING JOURNAL, October 1961. Whichever process is used, however, all the sulfuric acid which has not already been neutralized with ammonia has to be neutralized before disposal. This is usually effected by addition of lime or limestone and it results in the precipitation of calcium sulfate in admixture with the hydroxides of iron, aluminum and other metals. This precipitated material has no commercial value and has to be disposed of as waste. Moreover, the ammonium sulfate formed in the process is an undesirable constituent of liquid waste discharged as effluent and it also represents a loss of costly raw material unless the ammonia is recovered. This is usually done in the conventional manner of boiling with lime to volatize ammonia, which is then condensed and recycled.
It is an objective of the present invention to provide a simple process for the recovery of vanadium from acid sulfate solutions such as those derived by sulfuric acid leaching of residues of the type described above.